U.S. patent application number 15/760999 was filed with the patent office on 2018-09-20 for stiffness tuning and dynamic force balancing rotors of downhole drilling motors.
This patent application is currently assigned to Halliburton Energy Services, Inc.. The applicant listed for this patent is Halliburton Energy Services, Inc.. Invention is credited to Victor GAWSKI, John K. SNYDER, Michael John STRACHAN.
Application Number | 20180266181 15/760999 |
Document ID | / |
Family ID | 58797691 |
Filed Date | 2018-09-20 |
United States Patent
Application |
20180266181 |
Kind Code |
A1 |
GAWSKI; Victor ; et
al. |
September 20, 2018 |
STIFFNESS TUNING AND DYNAMIC FORCE BALANCING ROTORS OF DOWNHOLE
DRILLING MOTORS
Abstract
A method of manufacturing a power unit for a downhole drilling
motor includes fabricating a stator that provides two or more
stator lobes that define an internal profile, and fabricating a
rotor that provides at least one rotor lobe that defines an
external profile that both rotates and precesses within the
internal profile during operation. At least one of an external
geometry and an internal geometry of the rotor along all or a
portion of the rotor may be varied to alter a stiffness and mass of
the rotor and thereby optimize stiffness and force balancing with
respect to the stator. The rotor may then be rotatably positioned
within the stator and thereby optimize the functioning and
reliability of the motor power unit, of the downhole drilling motor
assembly, of associated downhole drilling equipment, and to enhance
directional drilling tendency modelling and directional drilling
trajectory control.
Inventors: |
GAWSKI; Victor;
(Whitecairns, GB) ; SNYDER; John K.; (Bakersfield,
CA) ; STRACHAN; Michael John; (Conroe, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Halliburton Energy Services, Inc. |
Houston |
TX |
US |
|
|
Assignee: |
Halliburton Energy Services,
Inc.
Houston
TX
|
Family ID: |
58797691 |
Appl. No.: |
15/760999 |
Filed: |
March 17, 2016 |
PCT Filed: |
March 17, 2016 |
PCT NO: |
PCT/US2016/022936 |
371 Date: |
March 16, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 4/02 20130101; F04C
14/20 20130101; F04C 2/1071 20130101; F03B 13/02 20130101; E21B
7/067 20130101 |
International
Class: |
E21B 4/02 20060101
E21B004/02; E21B 7/06 20060101 E21B007/06; F04C 14/20 20060101
F04C014/20; F04C 2/107 20060101 F04C002/107 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2015 |
US |
PCT/US2015/062988 |
Claims
1. A method of manufacturing a power unit for a downhole drilling
motor, comprising: fabricating a stator that provides two or more
stator lobes that define an internal profile; fabricating a rotor
that provides at least one rotor lobe that defines an external
profile that precesses within the internal profile during
operation; varying at least one of an external geometry and an
internal geometry of the rotor along all or a portion of the rotor
to alter a stiffness of the rotor and thereby optimize stiffness
with respect to the stator; and rotatably positioning the rotor
within the stator.
2. The method of claim 1, wherein varying the external geometry of
the rotor comprises altering a dimension of the external
profile.
3. The method of claim 1, wherein varying the external geometry of
the rotor comprises securing one or more stiffening elements to the
external profile.
4. The method of claim 1, wherein the rotor defines a rotor bore
and varying the internal geometry of the rotor comprises defining
one or more internal recesses in the rotor bore.
5. The method of claim 1, wherein the rotor defines a rotor bore
and varying the internal geometry of the rotor bore comprises
defining one or more profiles in the rotor bore that extend axially
along all or a portion of the rotor bore according to a
function.
6. (canceled)
7. The method of claim 1, wherein the rotor defines a rotor bore
and varying the internal geometry of the rotor comprises
positioning one or more stiffening elements within the rotor
bore.
8. The method of claim 7, further comprising selectively
positioning the one or more stiffening elements within the rotor
bore to optimize the stiffness with respect to the stator.
9. (canceled)
10. The method of claim 1, wherein the rotor includes at least one
of a rotor mandrel and a rotor sleeve, and wherein varying at least
one of the external geometry and the internal geometry of the rotor
comprises: varying a stiffness of at least one of the rotor, the
rotor mandrel, and the rotor sleeve relative to a stiffness of a
stator housing and a stator lining.
11. The method of claim 1, wherein the rotor comprises a rotor
sleeve that defines the external profile and a rotor mandrel
positioned within the rotor sleeve and defining a rotor bore, the
method further comprising varying a geometry of an interface
between the rotor mandrel and the rotor sleeve to alter the
stiffness of the rotor and thereby optimize a force balancing with
respect to the stator.
12. The method of claim 1, wherein the rotor comprises a rotor
sleeve that defines the external profile and a rotor mandrel
positioned within the rotor sleeve and defining a rotor bore, the
method further comprising mass balancing at least one of the rotor,
the rotor sleeve, and the rotor mandrel.
13. A power unit for a downhole drilling motor, comprising: a
stator that provides two or more stator lobes that define an
internal profile; and a rotor rotatably positioned within the
stator and providing at least one rotor lobe that defines an
external profile that precesses within the internal profile during
operation, wherein at least one of an external geometry and an
internal geometry of the rotor is varied along all or a portion of
the rotor to alter a mass of the rotor and thereby optimize force
balancing with respect to the stator.
14. The power unit of claim 13, wherein the external geometry of
the rotor is varied by altering a dimension of the external
profile.
15. The power unit of claim 13, wherein one or more stiffening
elements are secured to the external profile to vary the external
geometry of the rotor.
16. The power unit of claim 13, wherein the rotor defines a rotor
bore and one or more internal recesses are defined in the rotor
bore to vary the internal geometry of the rotor.
17. The power unit of claim 13, wherein the rotor defines a rotor
bore and one or more profiles are defined in the rotor bore and
extend axially along all or a portion of the rotor bore according
to a function to vary the internal geometry of the rotor.
18. (canceled)
19. The power unit of claim 13, wherein the rotor defines a rotor
bore and one or more stiffening elements are positioned within the
rotor bore to vary the internal geometry of the rotor.
20. The power unit of claim 19, wherein the one or more stiffening
elements comprise weighting elements that optimize the force
balancing with respect to the stator.
21. The power unit of claim 13, wherein the rotor comprises a rotor
sleeve that defines the external profile and a rotor mandrel is
positioned within the rotor sleeve.
22. The power unit of claim 21, wherein a geometry of an interface
between the rotor mandrel is varied to alter a stiffness of the
rotor with respect to a stiffness of the stator.
23. (canceled)
24. (canceled)
25. The power unit of claim 13, wherein the stator comprises a
stator housing that defines the internal profile and a stator
coating applied to the internal profile and comprising an elastomer
or a rubber.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of International
Application. No. PCT/US2015/62988, filed Nov. 30, 2015, and claims
priority thereto.
BACKGROUND
[0002] Positive displacement downhole drilling motors (also
referred to as "PDM's" and "mud motors") used in the oil and gas
industry are tubular assemblies consisting generally of a
progressing cavity power unit, a transmission unit, and an output
driveshaft. Downhole drilling motors are typically connected
directly to a drill bit at the output end of the motor driveshaft
and operate on the reverse application of the Moineau progressing
cavity pump principle. In this type of motor design, a stator and
rotor combination of the power unit converts hydraulic energy of a
pressurized circulating fluid to mechanical energy of the rotating
output driveshaft.
[0003] The rotor and stator are typically of lobed design, with the
rotor and stator having similarly lobed profiles. In general, the
power unit may be categorized based upon the number of lobes and
effective stages. The rotor is generally formed from steel or
stainless steel and has one less lobe than the stator, which is
often lined with an elastomer layer. The rotor and stator lobes
exhibit a helical configuration with one stage equating to the
linear distance of a full 360.degree. wrap of the stator helix. The
complementary lobes of the rotor and the stator and the associated
helix angles are designed such that the rotor and the stator seal
at discrete intervals, which results in the creation of axial fluid
chambers or cavities that are filled by the pressurized circulating
fluid. The action of the pressurized circulating fluid causes the
rotor to rotate and precess within the stator. Motor output torque
is directly proportional to the differential pressure developed
across the rotor and the stator during operation. In drilling
operations, the rotation speed of an associated drill bit is
directly proportional to the circulating fluid flow rate between
the rotor and the stator.
[0004] Downhole drilling motors typically form part of a bottom
hole assembly (BHA) included in a string of drill pipe (i.e., a
drill string) extended downhole from a surface location, such as a
drilling rig or platform. The external physical loading of the
downhole drilling motor is directly influenced by the torsional and
compressive or tensile loads applied from the surface via the drill
string, upon the relative geometries of the wellbore and the
drilling assembly components, and upon the physical characteristics
of the motor itself, including the use of stabilizers (if any) and
associated drilling equipment included in the BHA. Additionally,
the downhole environment (e.g. geothermal temperature), drilling
fluid characteristics and interaction between the drill bit and the
underlying formation can influence the downhole drilling motor.
[0005] External motor loading can result in bending of the rotor
and the stator and, since the stator typically exhibits a lower
stiffness than the rotor, the stator will tend to bend first and
the stiffer rotor will impart irregular mechanical loading into the
elastomer lining of the stator. More particularly, as the rotor
rotates within the stator during use, centrifugal loading is
created due to the mass of the rotor, and such centrifugal loading
is resisted by the elastomer lining along all or a portion of the
length of the rotor. Rotation of a transmission unit coupled to the
rotor also generates centrifugal loading that is transferred to the
rotor and is also resisted by the elastomer lining. Consequently,
the elastomer lining performs a radial bearing function for the
rotating mass of the rotor and a portion of the mass of the
transmission unit.
[0006] Such bending and centrifugally induced loading can reduce
fluid sealing efficiency and power production, accelerate stator
elastomer degradation, increase rotor and stator lobe profile wear,
can potentially cause significant damage to associated components
within the downhole drilling motor, and can negatively affect
directional drilling control. Directional drilling trajectory
objectives are planned for by accurately mechanically modelling the
BHA, to an individual drilling tool sub-component level, and as an
overall system, relative to the downhole conditions and surface
applied operating parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The following figures are included to illustrate certain
aspects of the present disclosure, and should not be viewed as
exclusive embodiments. The subject matter disclosed is capable of
considerable modifications, alterations, combinations, and
equivalents in form and function, without departing from the scope
of this disclosure.
[0008] FIG. 1 depicts an exemplary well system that may employ the
principles of the present disclosure.
[0009] FIG. 2 is a side elevation view partially in cutaway of an
exemplary downhole drilling motor.
[0010] FIG. 2A is a view of an exemplary embodiment of a downhole
drilling motor that schematically depicts the geometric and
mechanical interaction of the motor within a wellbore.
[0011] FIGS. 3A-3D are cross-sectional end views of various
exemplary embodiments of the power unit of FIG. 2.
[0012] FIGS. 4A-4C depict cross-sectional side views of exemplary
embodiments of the rotor of FIGS. 2 and 3A-3D in accordance with
the principles of the present disclosure.
[0013] FIG. 5 is a cross-sectional end view of an exemplary
embodiment of the power unit of FIG. 2 that schematically depicts
the geometric interaction between the rotor and the stator.
DETAILED DESCRIPTION
[0014] The present disclosure is related to downhole drilling
motors and, more particularly, to tuning and balancing the
stiffness and dynamic forces of the rotor included in a downhole
drilling motor.
[0015] Embodiments described herein provides methods to optimize
and otherwise mitigate the effects of dynamic internal and external
mechanical loading of a rotor-stator combination of a downhole
drilling motor. This may be accomplished by optimizing the
stiffness of the rotor to thereby compliment the stiffness
exhibited by the stator, which may benefit performance and
component reliability of the downhole drilling motor and enhance
directional drilling control. This may also be accomplished by
optimizing and balancing the centrifugal loading caused by the
rotating mass of the rotor, which may also benefit performance and
component reliability of the downhole drilling motor, but also
benefit the performance and reliability of associated downhole
drilling equipment positioned above or below the motor. Such
associated downhole drilling equipment can include, but is not
limited to, a Measurement While Drilling "MWD" tool, a Rotary
Steerable System "RSS," and Weight On Bit "WOB," Torque On Bit
"TOB," and Bending On Bit "BOB" measurement tools.
[0016] FIG. 1 depicts an exemplary well system 100 that may employ
the principles of the present disclosure. More particularly, the
well system 100 may include an offshore, semi-submersible oil and
gas production platform 102 centered over a submerged oil and gas
formation 104 located below a sea floor 106. A subsea conduit or
riser 108 extends from a deck 110 of the platform 102 to a wellhead
installation 112 that may include one or more blowout preventers
114. The platform 102 has a hoisting apparatus 116 and a derrick
118 for raising and lowering tubular lengths of drill pipe, such as
a drill string 120.
[0017] A wellbore 122 extends through the various earth strata
toward the subterranean hydrocarbon bearing formation 104 and the
drill string 120 is extended within the wellbore 122. At its distal
end, the drill string 120 includes a bottom hole assembly (BHA) 123
that includes a drill bit 124 and a downhole drilling motor 126,
also referred to as a positive displacement motor ("PDM") or "mud
motor".
[0018] As explained in more detail below, circulating fluid is
pumped through an interior fluid passageway of the drill string 120
to the downhole drilling motor 126, which converts the hydraulic
energy of the circulating fluid to mechanical energy in the form of
a rotating rotor. The rotor is coupled to the drill bit 124 via a
transmission unit and output driveshaft to cause rotation of the
drill bit 124, and thereby allows the wellbore 122 to be
extended.
[0019] Even though FIG. 1 depicts a vertical wellbore 122 being
drilled, it should be understood by those skilled in the art that
the downhole drilling motor 126 is equally well suited for use in
horizontal or deviated wellbores. It will also be understood by
those skilled in the art that the use of directional terms such as
above, below, upper, lower, upward, downward and the like are used
in relation to the illustrative embodiments as they are depicted in
the figures, the upward direction being toward the top of the
corresponding figure and the downward direction being toward the
bottom of the corresponding figure. In addition, even though FIG. 1
depicts an offshore operation, it should be understood by those
skilled in the art that the downhole drilling motor 126 is equally
well suited for use in onshore operations.
[0020] FIG. 2 depicts an exemplary embodiment of the downhole
drilling motor 126 of FIG. 1, according to one or more embodiments.
As illustrated, the downhole drilling motor 126 (hereafter "the
motor 126") may be coupled to a lower end of the drill string 120
and may include a power unit 202, a transmission section 204 housed
within an angularly `offset housing` 204a, a bearing section 206, a
stabilizer section 208 and a drill bit section 210 that includes
the drill bit 124. The drill string 120 includes an interior fluid
passageway 212 for transporting a circulating fluid to an internal
fluid passageway 213 defined in the power unit 202 to drive the
motor 126. In some cases, the circulating fluid may comprise
drilling fluid (also known as "mud"), which may be used to cool the
drill bit 124 and carry cuttings back to the surface. In other
cases, the motor 126 may be configured to accommodate other types
of circulating fluids, including water, air, and foam while
producing the output characteristics required to achieve effective
drilling of underlying subterranean formations (i.e., the formation
104 of FIG. 1).
[0021] The power unit 202 includes an internally profiled stator
214 and an externally profiled rotor 216, various configurations of
which will be discussed in detail below. The transmission section
204 may include one or a pair of articulated connections 218, 220
and a transmission shaft 222, which work together to eliminate the
eccentric motion of the rotor 216 while transmitting torque and
downthrust. The hydraulic energy of the pressurized circulating
fluid flowing through the power unit 202 is converted to mechanical
energy via the rotating and precessing rotor 216. The action of the
circulating fluid across the cross-section of the rotor 216, which
is effectively sealed within the stator 214, also produces a
hydraulic downthrust on the rotor 216.
[0022] The transmission shaft 222 can be a multi-element assembly
or a one-piece flexible shaft (e.g., a torsion rod), and may be
connected to the lower end of the rotor 216 at the upper connection
218 and connected to an output driveshaft 226 at the lower
connection 220. The upper end of the transmission shaft 222 may be
under the influence of the eccentric movement of the rotor 216
within the stator 214, while the lower end of the transmission
shaft 222 rotates concentrically with the driveshaft 226 and
thereby transmits torque and downthrust to the driveshaft 226. The
transmission shaft 222 operates to eliminate the eccentric motion
of the rotor 216 and the geometric effects of a fixed or rig-site
adjustable angularly offset housing 204a (i.e., a "bent housing")
within the motor 126. Accordingly, the transmission shaft 222
allows for the optimum longitudinal axis relationship of the rotor
216 to the stator 214, and thereby ensuring efficient
rotor-to-stator sealing and efficiency while minimizing wear on the
rotor 216 and the stator 214.
[0023] While a particular transmission section has been illustrated
in FIG. 2, those skilled in the art will readily appreciate that
other types of transmissions could be used in conjunction with the
present disclosure, including transmissions having multi-element
designs that utilize universal couplings, for example.
[0024] The bearing section 206 may include a variety of bearings
224, such as thrust bearings and radial bearings. The thrust
bearings may operate to support the downthrust from the
transmission section 204 and the reactive upward loading from the
applied weight on the drill bit 124, and the radial bearings may
operate to absorb lateral side loading of the driveshaft 226 that
extends from the transmission section 204. The driveshaft 226 is
operatively coupled to and otherwise transmits both axial and
torsional loading to the drill bit 124.
[0025] The external stabilizer section 208 includes a plurality of
blades or pads that provide both stabilization and protection to
the motor 126. The driveshaft 226 includes a drill bit connector
228 that threadably receives the drill bit 124 to couple the
driveshaft 226 with the drill bit 124.
[0026] FIG. 2A depicts another exemplary embodiment of the downhole
drilling motor 126 of FIG. 1, according to one or more additional
embodiments. More particularly, FIG. 2A schematically illustrates
the various mechanical loading forces that may be applied to the
motor 126 while drilling a directional wellbore 230. As
illustrated, the drill string 120 conveys compression or tension to
the motor 126, and as it rotates, the drill string 120 further
conveys torque to the motor 126. During operation, the power unit
120 is acted upon by compression, tension, and alternating bending
forces. A reactive torque is generated as the drill bit 124 engages
the underlying formation, resulting from weight being applied to
the drill bit 124 and the motor 126 from a surface location via the
drill string 120.
[0027] Downhole motor drilling is undertaken with the drill string
120 stationary and while rotating. When there is no rotation of the
drill string 120 this is referred to as "slide mode" drilling.
While drilling in slide mode, the motor 126 is always pumped
through or operational before the drill bit 124 engages the bottom
of the borehole. When weight-on-bit (WOB) is applied by the drill
string 120 during on-bottom drilling or forward-reaming, the
compressive load from the drill string 120 interacts with the
inherent stiffness of the motor 126. The motor housings and
driveshaft 226 (FIG. 2) tend to be compressed by the applied WOB.
In some applications the compressive load can be sufficient to bend
the stator housing (the stator 214 is positioned near the motor
bend point), this produces additional loading between the mated
rotor and stator lobe forms and any rotor coating or stator lining.
The wall of the wellbore 230 also produces a physical restriction
to movement of the motor 126. Tensile load is imparted to the side
of the housings about the inside of the bend in the motor 126,
thereby tending to reduce compression in the housings about the
inside of the bend. When the motor 226 is pumped-through/operated
and during rotation of the rotor 216 and the interconnected
transmission and driveshaft 226, bending loads in the rotor 216,
the transmission section 204, and driveshaft 226 fluctuate. The
bending load fluctuations produce additional loading between the
mated rotor and stator lobe forms and any coatings or stator
lining. The driveshaft 226 bears both the rotor hydraulic
downthrust compressive loading and the reaction to the WOB
compressive loading (these act in opposite directions), plus torque
generated by the motor 126 in reaction to drill bit 124-formation
interactions (at the drill bit 124 face and periphery). Driveshaft
226 compressive and torsional loading fluctuates during its
rotation and depends on motor 126 geometry and stiffness, wellbore
geometry, drill bit 124-formation interactions (at the drill bit
124 face and periphery) and any BHA-wellbore 230 interactions. When
circulating in the same position with the drill bit 124 off-bottom,
the wall of the wellbore 230 wall can produce a physical
restriction. More particularly, the interaction of the wall of the
wellbore 230 and the inherent stiffness of the motor 126 results in
tensile loading of the side of the housings about the inside of the
bend in the motor 126. Tensile loading of the stator housing
promotes bending of the stator housing, which produces additional
loading between the mated rotor and stator lobe forms and any
coatings or stator lining.
[0028] When back-reaming, upwards-pull or tensile load is applied
to the drill string 120. The wellbore 230 wall produces a physical
restriction and imparts tensile load to the side of the housings
about the inside of the bend in the motor 126. Any drill bit
124-formation interactions (at the drill bit 124 periphery) and any
BHA-wellbore interactions increase the tensile loads imparted to
the motor 126 and the BHA. Tensile loading of the stator housing
promotes bending of the stator housing, which produces additional
loading between the mated rotor and stator lobe forms and any rotor
coating or stator lining. When the motor 126 is
pumped-through/operated, during a rotation of the rotor 216 and the
interconnected transmission and driveshaft 226, the bending loads
in the rotor 216, the transmission section, and the driveshaft 226
fluctuate. The bending load fluctuations produce additional loading
between the mated rotor and stator lobe forms and any rotor coating
or stator lining. The driveshaft 226 bears the rotor hydraulic
downthrust compressive loading and opposing tensile load applied
via the drill string 120, plus torque generated by the motor 126 in
reaction to drill bit 124-formation interactions. Driveshaft 226
net longitudinal loading can be compressive or tensile. Driveshaft
226 longitudinal and torsional loadings fluctuate during its
rotation, depending on motor geometry, wellbore geometry, drill
bit-formation interactions and any BHA-wellbore interactions.
[0029] When drilling in rotary mode, the motor 126 is always pumped
through or operational before the drill string 120 is rotated and
before the drill bit 124 engages the bottom of the borehole. When
the drill string 120 rotates, drill string 120 torque and WOB are
applied to the motor 126 during drilling or forward reaming, WOB
compressive load interacts with the inherent stiffness of the motor
126. The motor 126 tends to be compressed by the applied WOB, the
compression in the side of the motor housings about the inside of
the bend tends to reduce as the hole wall produces a physical
restriction. A resulting tensile load is imparted into the housings
about the inside of the bend in the motor 126. In some
applications, the compressive load can be sufficient to bend the
stator housing, which produces additional loading between the mated
rotor and stator lobe forms and any rotor coating or stator lining.
The hole wall provides a physical restriction to motor 126
movement; tensile load is imparted to the side of the housings
about the inside of the motor 126 bend, tending to reduce any
compression in the housings about the inside of the bend. Torque
delivered from the drill string 126 is borne by the stator 214, the
rotor 216 and interconnected transmission and driveshaft 226. Motor
126 housing loading fluctuates during motor 126 rotation, depending
on motor 126 geometry, wellbore 230 geometry, drill bit-formation
interactions (at bit face and periphery) and any BHA-wellbore
interactions. When the motor 126 is also pumped-through/operated,
during a rotation of the rotor 216 and interconnected transmission
and driveshaft 226, the bending loads in the rotor 216,
transmission and driveshaft 226 fluctuate. The bending load
fluctuations produce additional loading between the mated rotor and
stator lobe forms and any coatings or stator lining. The driveshaft
226 bears both the rotor 216 hydraulic downthrust compressive
loading and the reaction to the WOB compressive loading (these act
in opposite directions), plus torque delivered from the drill
string 120 and torque generated by the motor 126 in reaction to
drill bit 124-formation interactions. The driveshaft 226
compressive and torsional loading fluctuates during its rotation,
depending on the motor 126 geometry and stiffness, wellbore 230
geometry, drill bit 124-formation interactions (at the drill bit
124 face and periphery) and any BHA-wellbore interactions.
[0030] When rotating the drill string 120 and circulating in the
same position with the drill bit 124 off-bottom, the inherent
stiffness of the motor 126 and the drill string 120 torque
interact. The hole wall produces a physical restriction, a
resulting tensile load is imparted into the side of the housings
about the inside of the bend in the motor 126. The side of the
housings on the inside of the bend tend to be in tension. Tensile
loading of the stator housing promotes bending of the stator
housing, this produces additional loading between the mated rotor
and stator lobe forms and any coatings or stator lining. Torque
delivered from the drill string 120 is borne by the stator 214, the
rotor 216, and interconnected transmission and driveshaft 226. The
motor 126 housing loading fluctuates during motor 126 rotation
depending on the motor 126 geometry, the wellbore 230 geometry, and
any BHA-wellbore interactions. When the motor 126 is also
pumped-through/operated, during rotation of the rotor 216 and
interconnected transmission and driveshaft 226, the bending loads
in the rotor 216, transmission, and driveshaft 226 fluctuate. The
bending load fluctuations produce additional loading between the
mated rotor and stator lobe forms and any rotor coating or stator
lining. There are additional loadings caused by the torque
delivered from the drill string 120. The driveshaft 226 bears the
rotor 216 hydraulic downthrust compressive loading, plus torque
delivered from the drill string 120 and generated by the motor 126
in reaction to the drill bit 124-formation interactions. The
driveshaft 226 compressive and torsional loading fluctuates during
its rotation, depending on the motor 126 geometry and stiffness,
the wellbore 230 geometry, the drill bit 124-formation
interactions, and any BHA-wellbore interactions.
[0031] When back-reaming with drill string 120 rotation, the drill
string 120 torsion and tensile load are applied to the motor 126,
and the applied tensile load interacts with the inherent stiffness
of the motor 126. The motor 126 housings tend to be loaded in
tension by the applied upwards pull, plus the hole wall produces a
restriction and resulting tensile load into the side of the
housings about the inside of the bend in the motor 126. Tensile
loading of the stator housing promotes bending of the stator
housing, this produces additional loading between the mated rotor
and stator lobe forms and any coatings or stator lining. Torque
delivered from the drill string 120 is borne by the stator housing
and lobe form, the rotor 216 and its lobe form, the interconnected
transmission, and the driveshaft 226. The motor 126 housing loading
fluctuates during motor 126 rotation, depending on the motor 126
geometry, the wellbore 230 geometry, the drill bit 124-formation
interactions and any BHA-wellbore interactions. When the motor 126
is also pumped-through/operated, during rotation of the rotor 216
and interconnected transmission and driveshaft 226, the bending
loads in the rotor 216, transmission and driveshaft 226 fluctuate.
The bending load fluctuations produce additional loading between
the mated rotor and stator lobe forms and any coatings or stator
lining. There are additional loadings caused by the torque
delivered from the drill string 120. The driveshaft 226 bears the
rotor 216 hydraulic downthrust compressive loading, any tensile
loading caused by any wellbore 230 interactions at the periphery of
the drill bit 124, plus torque delivered from the drill string 120
and generated by the motor 126, in reaction to drill bit
124-formation interactions. The driveshaft 226 net longitudinal
loading can be compressive or tensile. The driveshaft 226
longitudinal and torsional loadings fluctuate during its rotation,
depending on the motor 126 geometry, the wellbore 230 geometry, the
drill bit 124-formation interactions and any BHA-wellbore
interactions.
[0032] FIGS. 3A-3D are cross-sectional end views of various
exemplary embodiments of the power unit 202 as taken along the
indicated lines in FIG. 2, according to embodiments of the present
disclosure. As illustrated, the power unit 202 in each embodiment
includes the stator 214 and the rotor 216. The stator 214 provides
a multi-staged, profiled inner surface that defines a plurality of
stator lobes 302 that have a helical configuration wherein each
stage is defined by the linear distance of one full 360.degree.
wrap of the stator helix. In the illustrated embodiments, the
stator 214 has seven lobes 302, but it will be appreciated that the
power unit 202 may incorporate the use of more or less than seven
lobes 302, without departing from the scope of the present
disclosure.
[0033] The number of stator lobes 302 used in the power unit 202
may be determined based upon factors including the desired speed of
rotation and the desired torque. Power units of the same diameter
but having fewer stator lobes generally operate at higher speeds
and deliver lower torque per unit length as compared to power units
having a greater number stator lobes that tend to operate at lower
speeds but deliver greater torque. In some embodiments, the power
unit 202 may include between two and ten stator lobes 302, but the
power unit 202 may alternatively include more than ten stator lobes
302, without departing from the scope of the present
disclosure.
[0034] The rotor 216 in FIGS. 3A-3D has a profiled outer surface
that closely matches the profiled inner surface of the stator 214
to provide a close fitting relationship, such as an interference
(compression) or a clearance cross-sectional fit. The profiled
outer surface of the rotor 216 defines a plurality of rotor lobes
304 that, similar to the stator lobes 302, exhibit a helical
configuration. In the illustrated embodiments, the rotor 216 has
six lobes 304, but it will be appreciated that the power unit 202
may incorporate the use of more or less than six lobes 304, without
departing from the scope of the present disclosure. The number of
rotor lobes 304 used in the power unit 202 will be determined based
upon the number of stator lobes 302, with the number of rotor lobes
304 being one less than the number of stator lobes 304. For
example, if the number of stator lobes 302 is (n), then the number
of rotor lobes 304 will be (n-1).
[0035] The profiles of the lobes 302, 304 may be similar, but the
effective operating diameter of the lobes 302, 304 is different. As
described in more detail below, for instance, the stator lobes 302
are set on a larger operating diameter than that of the rotor lobes
304. Modification of the numbers of lobes 302, 304 provides for
variation of the input and output operating characteristics of the
power unit 202 to accommodate different drilling operations
requirements. The rotor 216 is inserted into the stator 214 during
assembly of the drilling motor power unit 202. The effective
operating stages of the power unit 202 depend on the longitudinal
length along which the rotor and stator lobed profiles directly
interact, mate, or mesh.
[0036] In FIG. 3A, the stator 214 includes a stator housing 306 and
a stator sleeve 308 disposed within the stator housing 306. The
stator housing 306 may be formed from a metal, such as a ferrous
metal including steels and stainless steels. Alternatively, the
stator housing 306 may be formed of other rigid, non-metallic
materials including, but not limited to, carbon fiber, a polymer
composite, or other rigid composite materials. The stator housing
306 has an inner surface sized to the receive stator sleeve 308,
and the stator sleeve 308 may be coupled to the stator housing 306
using a system of tapers, orientation keys, matched surfaces,
threaded components or the like that are designed to torsionally
and longitudinally support the stator sleeve 308 within the stator
housing 306 during operation. The stator sleeve 308 may be formed
from a malleable metal, such as steel alloys, aluminum, aluminum
alloys, copper, copper alloys (e.g., beryllium copper alloys),
bronze, bronze alloys (e.g., magnesium bronze, aluminum bronze,
etc.), or similar metals.
[0037] In some embodiments, as illustrated, the rotor 216 may
comprise a solid, elongate structure milled and otherwise formed
from a metal such as a ferrous metal including steels and stainless
steels. In such embodiments, the helical lobes 304 of the rotor 216
may be precision formed using multi-axis milling to tight axial and
radial tolerances. Alternatively, the rotor 216 may be manufactured
via swaging (cold working) or a pressure forming technique. In some
embodiments, the outer surface of the rotor 216 may be treated with
a treatment process such as, but not limited to, salt bath
nitriding, gas nitriding, plasma nitriding, ion nitriding, ion
plating, inductive hardening, anodizing, thermal metallic spraying,
or the like. Such treatment processes modify surface properties of
the rotor 216, such as maximizing wear and corrosion resistance,
but not the surface geometry. In operating the power unit 202, the
rotor 216 rotates and precesses within the stator 214 and, in at
least one embodiment, there may be metal-to-metal contact between
the outer surface of the rotor 216 and the inner surface of the
stator 214, which are formed from dissimilar metals.
[0038] Alternatively, the stator sleeve 308 may be formed from a
polymer. For example, the stator sleeve 308 may be made of
polychloroprene rubber (CR), natural rubber (NR), polyether
eurethane (EU), styrene butadiene rubber (SBR), ethylene propylene
(EPR), ethylene propylene diene (EPDM), a nitrile rubber, a
copolymer of acrylonitrile and butadiene (NBR), carboxylated
acrylonitrile butadiene (XNBR), hydrogenated acrylonitrile
butadiene (HNBR), commonly referred to as highly-saturated nitrile
(HSN), carboxylated hydrogenated acrylonitrile butadiene (XHNBR),
hydrogenated carboxylated acrylonitrile butadiene (HXNBR) or
similar material.
[0039] When made of an elastomer or rubber, the stator sleeve 308
may be injection molded with detailed attention being given to
elastomer composition, uniformity, bond integrity and lobe 302
profile accuracy. In some embodiments, the stator sleeve 308 may be
injection molded into or directly sprayed onto the stator housing
306, and the bore of the stator housing 306 may have a bonding
agent applied thereto prior to the injection molding or spraying
process. In such embodiments, as the rotor 216 rotates within the
stator 214 during use, centrifugal load is created due to the mass
of the rotor 216, and this is resisted by the elastomeric stator
sleeve 308 along the length of the rotor 216. The stator sleeve 308
thus performs a radial bearing function with respect to the
rotating mass of the rotor 216.
[0040] In FIG. 3B, a stator coating 310 may be applied to the
stator housing 306 and the rotor 216 may comprise a tubular
structure that defines and otherwise provides a rotor bore 312. The
rotor bore 312 may provide a bypass passageway through the rotor
216 for circulating fluids and may help prevent rotor stall under
certain conditions.
[0041] As illustrated, the stator housing 306 provides a profiled
inner surface that defines the stator lobes 302 that receive the
stator coating 310 thereon. In some embodiments, the stator coating
310 may comprise a polymer, such as any of the rubbers or
elastomers mentioned herein. The internally profiled bore of the
stator 214 lined with a uniform thickness elastomer stator coating
310 may reduce elastomer flex and cyclic load levels. The stator
coating 310 may also increase operating pressure and output power
capacity and reduce heat generation within the stator 214.
[0042] In other embodiments, the stator coating 310 may comprise a
metal coating, which may be applied using a vapor deposition
process, a metallizing process, an arc spraying process, a
thermospray process, a flame spray process, a plasma spray process,
a high velocity oxy-fuel process, or the like. In such embodiments,
the stator coating 310 may be formed from a pure metal, a metal
oxide, a metal alloy (e.g., stainless steel, carbon steel, etc.),
nickel, a nickel alloy (e.g., nickel-chrome, nickel-chrome-boron,
cobalt-nickel-chrome), aluminum, an aluminum alloy or aluminum
oxide, bronze, a bronze alloy (e.g., magnesium bronzes and aluminum
bronzes), copper, a copper alloy (e.g., beryllium copper alloy),
molybdenum, tin, zinc, a zinc alloy, MONEL.RTM., HASTELLOY.RTM.,
tungsten carbide, tungsten carbide-nickel, tungsten carbide-cobalt,
chromium carbide, chromium oxide, titanium or titanium oxide,
mirconium oxide, cobalt-molybdenum-chromium, or similar materials.
Moreover, the surface of the stator coating 310 may receive a
treatment process, such as any of the treatment processes mentioned
herein.
[0043] In FIG. 3C, the stator 214 is a solid metal stator and the
rotor 216 includes a tubular rotor mandrel 314 that defines and
otherwise provides the rotor bore 312. More particularly, the rotor
216 may be characterized as a rotor sleeve 216a that comprises a
tubular structure defining a central bore 316 sized to receive the
rotor mandrel 314. The rotor mandrel 314 may be formed from a
metal, such as a ferrous metal including steels and stainless
steels, and may be coupled to the rotor 216 at the central bore 316
via a variety of means. For instance, the rotor mandrel 314 may be
coupled to the rotor sleeve 216a using a system of complimentary
tapers, matched surfaces, orientation keys, and/or threaded
components designed to torsionally and longitudinally support the
rotor mandrel 314 within the central bore 316. Surfaces of one or
both of the stator 214 and the rotor 216 may receive a treatment
process, as discussed herein.
[0044] The rotor mandrel 314 may prove advantageous in allowing an
operator to selectively vary or otherwise tune the stiffness of the
combined rotor mandrel 214 and rotor sleeve 216a to compliment the
stiffness of the stator 214. As will be appreciated, this may
prevent accelerated wear and deterioration when the stator 214
flexes due to mechanical loading during operation and differences
in relative stiffness between the rotor 216 and the stator 214.
Moreover, the rotor mandrel 314 may help facilitate simplified
maintenance of the power unit 202 as the rotor sleeve may be
removed from the rotor mandrel 314 for replacement, repair, or
refurbishment.
[0045] In FIG. 3D, the stator 214 is again a solid metal stator and
the rotor 216 again includes the rotor mandrel 314 secured within
the central bore 316 and defining the rotor bore 312. The rotor 216
may further include a rotor coating 318 defined on or otherwise
formed about the profiled outer surface. In some embodiments, the
rotor coating 318 may comprise a polymer, such as any of the
rubbers or elastomers listed herein. In other embodiments, however,
the rotor coating 318 may comprise a metal coating applied to the
profiled outer surface of the rotor 216 via a vapor deposition
process, a metallizing process, an arc spraying process, a
thermospray process, a flame spray process, a plasma spray process,
a high velocity oxy-fuel process or the like. In such embodiments,
the rotor coating 318 may comprise any of the metals or metal
alloys listed herein for the stator coating 310. Moreover, in at
least one embodiment, surfaces of one or both of the rotor coating
318 and the stator 214 may receive a treatment process, such as any
of the treatment processes mentioned herein.
[0046] While the power unit 202 is shown in FIGS. 3A-3D in specific
embodiments having specific component parts, it is noted that any
of the rotors 216 described above can operate with any of the
stators 214 described above, without departing from the principles
of the present disclosure. Accordingly, components of one rotor 216
design or stator 214 design may be used together or removed from
another rotor 216 design or stator 214 design, without departing
from the principles of the present disclosure. For example, a
stator 214 having a stator housing 306 and stator sleeve 308 could
also have a stator coating 310, in keeping with the scope of the
present disclosure.
[0047] Referring again to FIG. 1, with continued reference to FIGS.
2 and 3A-3D, when the drill bit 124 is lowered onto the formation
104 and weight (compression load) is applied to the motor 126 from
surface via the drill string 120, an increased operating
differential pressure is produced across the power unit 202 and an
increased amount of output torque results. The output rotation is
dependent on the amount of circulating fluid being pumped through
the motor 126. Drilling operations can be undertaken with the drill
string 120 stationary or rotating. When rotating, the drill string
120 supplies additional torque and rotation to the motor 126. This
combines with the output torque and rotation of the motor 126, the
cumulative torque and rotation being supplied to the drill bit
124.
[0048] The motor 126 can form part of a bottom hole assembly (BHA)
used, among other things, for directional drilling. The BHA enables
the inclination and direction of the drilled wellbore 122 to be
controlled as desired to meet geologic target objectives. Moreover,
the offset or bent housing 204a (FIGS. 2 and 2A) within the motor
126 can be configured during workshop assembly or rig floor
adjustment to be geometrically (angularly) offset with respect to
the longitudinal axis of the motor 126. As a result, a bend is
placed at a specific location within the longitudinal length of the
motor 126 and the motor 126 is effectively no longer straight along
its full length. The bend, along with the stabilizing elements of
the stabilizing section 208 (FIG. 2) and separate components of the
BHA 123 positioned above and/or below the motor 126, and thereby
affects the tendency that the motor 126 has to drill in a specific
direction for a given formation type.
[0049] Holding the drill string 120 static or rotating, the
physical motor 126 configuration directly influences the direction
of the drilled wellbore 122. Varying the WOB (compression load)
which is applied to the motor 126 from surface influences the
tendency that the motor 126 has to either drill straight ahead or
"directionally" to the left, to the right, or to increase or
decrease the inclination of the wellbore 122. The surface applied
weight on bit loads can be significant and can exceed 100,000 lbf
in larger diameter wellbores, and the majority of this load is
applied to the downhole drilling motor 126. Drilling in a specific
direction or a series of directions facilitates the wellbore 122
reaching specific geologic targets, which can be horizontally
displaced from the platform 102, while negating the effects of
gravity on the drilling assembly and of any deviation tendencies
caused by the physical characteristics of the formation being
drilled.
[0050] The lobes 302, 304 (FIGS. 3A-3D) of the stator 214 and the
rotor 216, respectively, and the associated helix angles may be
configured so that the stator 214 and the rotor 216 seal at
discrete intervals. This results in the creation of axial fluid
chambers or cavities between the stator 214 and the rotor 216,
which are progressively filled by the pressurized circulating
fluid. As mentioned above, the action of the pressurized
circulating fluid causes the rotor 216 to rotate and precess within
the stator 214. The geometry of the lobes 302, 304 and the amount
of eccentric movement of the rotor 216 is designed to minimize
contact pressure, sliding friction, abrasion and vibration, thus
reducing wear on the stator 214 and the rotor 216.
[0051] The input and output power characteristics of the motor 126
can generally be considered to be a function of the number and
geometry of the lobes 302, 304, the helix angle of the lobes 302,
304, and the number of effective stages along the length of the
power unit 202. Within the specified motor 126 operating ranges,
the rotation speed of the drill bit 124 is directly proportional to
the circulating fluid flow rate between the rotor 216 and the
stator 214. Above the maximum specified operating differential
pressure (pressure per stage) of the power unit 202, fluid leakage
occurs between the seals created between the rotor 216 and the
stator 214, thereby reducing efficiency, output torque, and output
rotation. Excessive fluid leakage results in no rotation of the
drill bit 124 due to the rotor 216 becoming stationary, or stalling
in the stator 214.
[0052] Moreover, within the specified operating ranges of the motor
126, the output torque of the motor 126 is directly proportional to
the differential pressure developed across the rotor 216 and the
stator 214. If the motor 126 is operated above the maximum
specified torque production values, there can be a tendency for
accelerated rotor 216 and stator 214 wear, degradation, and
stalling. The power developed by the rotor 216 and the stator 214
is directly proportional to both rotational speed and torque.
[0053] Designs of the rotor 216 and the stator 214 take account of
the various downhole operating parameters that may be present
during downhole drilling applications, including the effects of
circulating fluid weight/viscosity, temperature, solids content and
lost circulation materials content. Chemical constituents of
formation fluids and gases are also given detailed consideration
with respect to elastomers and other types of coatings applied to
either the rotor 216 or the stator 214 (e.g., the stator coating
310 and/or the rotor coating 318 of FIGS. 3B and 3D,
respectively).
[0054] The metrology system of the rotor 216 and the stator 214
ensures that the rotor 216 and the stator 214 are carefully
measured and accurately matched geometrically within small
tolerances to provide the optimum mating fit for planned downhole
operating conditions. In some embodiments, the mating fits and
geometries between the rotor 216 and the stator 214 are selected to
accommodate downhole (geothermal) motor drilling operating
temperatures, which can exceed 200.degree. C. (392.degree. F.). In
some embodiments, the mating fits and geometries between the rotor
216 and the stator 214 are selected to accommodate internally
generated motor heat, caused by the rotor 216 interacting with the
stator 214. Accordingly, allowance may be made for the effect of
motor component expansion caused by the formation temperature and
by internally generated heat. As will be appreciated, this avoids
motor 126 start-up problems, ensures acceptable output power
characteristics, and maximizes reliability and longevity of the
rotor 216 and the stator 214.
[0055] Mechanical loading of the motor 126 can negatively affect
its internal components through wear and through the application of
mechanical stress, which can promote internal heat generation,
fatigue cracking, and fracture of the components. Such loading also
tends to affect the directional drilling tendency and operational
control of the motor 126. Significant loads can be applied to the
internal components when they are downhole, the loads resulting
from operations such as drilling, reaming (back or forward) or
circulating fluid when the drill bit 124 is off-bottom. External
loading of the motor 126 is dependent on the torsion and
compression or tensile loads applied from surface via the drill
string 120, upon the relative geometries of the wellbore 122 and
the drilling assembly components, and upon the physical
characteristics of the motor 126, including the stabilizers of the
stabilizer section 208 (FIGS. 2 and 2A) and associated drilling
equipment located within the BHA.
[0056] The maximum outer diameter for the motor 126 is restricted
by the need to provide annular clearance in the wellbore 122 to
permit movement of the drilling assembly into and out of the well,
and to allow for the flow of circulating fluid and the movement of
formation cuttings. The inner dimensions of the motor 126 may be
configured to provide adequate housing strength in terms of the
compression, tension, torsion and bending loads applied during the
drilling process, and must physically accommodate the internal
functioning components of the motor 126.
[0057] The housing for the stator 214 (e.g., the stator housing
306) may be configured to accommodate the helical lobe form (e.g.,
the stator sleeve 308 and/or the stator coating 310) of the stator
214 and the rotor 216 that mates, rotates, and precesses within it.
When employing a relatively thick stator lining, it may be
necessary for the wall thickness of the stator housing to be
thinner as compared to a scenario where the circulating fluid was
passing through a plain housing bore, as is the case with
relatively thick walled drill collars. In some embodiments, the
outer surface of the stator housing 306 may be profiled (sometimes
referred to as "flexed") to selectively vary the stiffness of the
stator housing 306 and the motor 126.
[0058] When sufficient load (compression when drilling or
forward-reaming and tension when back-reaming, tripping in-hole or
pulling out of hole, etc.) is applied to bend the stator 214
relative to the rotor 216, the stiffness of the rotor 216 may
adversely load the lobe form of the rotor 216 and the stator 214,
any elastomer lining of the stator 214 or any rotor/stator coatings
(e.g., the stator coating 310 and/or the rotor coating 318). During
drilling operations with no drill string 120 rotation, one side of
the stator housing bore (e.g., the stator sleeve 308 and/or the
stator coating 310) may be adversely affected, bending of the rotor
216 and the stator housing 306 can be influenced by the BHA 123 and
the motor 126 physically interacting with the sides of the
wellbore. In drilling operations where the drill string 120
rotates, however, 360.degree. around the stator housing bore can be
adversely loaded. Alternating bending of the rotor 216 and the
stator housing 306 can occur due to the BHA 123 and the motor 126
being rotated by the drill string 120 and physically interacting
with the sides of the wellbore (FIG. 2A).
[0059] Excessive bending loads from the rotor 216 promote fluid
leakage between the seal generated between the rotor 216 and the
stator 214, thereby reducing output torque and rotation. The
loading applied to the stator housing bore, and therefore to the
stator lobes 302 (FIGS. 3A-3D), due to the stiffness of the rotor
216, tends to cause elastomer/coating wear and degradation, which
promotes elastomer or coating overheating, hardening and
cracking.
[0060] According to embodiments of the present disclosure, the
effects of dynamic internal and external mechanical loading of the
rotor-stator combination and of the elastomer lining on the stator
214 (if used) may be optimized and otherwise mitigated for drilling
operations. This may be accomplished by optimizing the stiffness of
the rotor 216 to thereby compliment the stiffness exhibited by the
stator 214, which may benefit performance and component reliability
of the motor 126, the reliability of associated drilling equipment
within the BHA, and directional drilling control. Similar benefits
may also be accomplished by optimizing and balancing the
centrifugal loading caused by the rotating mass of the rotor 216
and the coupled transmission assembly comprising the articulated
connections 218, 220 and the transmission shaft 222. This may also
benefit the performance and reliability of associated drilling
equipment within the BHA.
[0061] Referring to FIGS. 4A-4C, illustrated are cross-sectional
side views of exemplary embodiments of the rotor 216 in accordance
with the principles of the present disclosure. As illustrated, the
rotor 216 may include an elongate body 402 having a first or lower
end 404a and a second or upper end 404b opposite the first end
404b. The first end 404a may comprise a connection configured to
receive a corresponding mating component (not shown), such as part
of the transmission section 204 (FIG. 2) that allows the rotor 216
to rotate with respect to the stator 214 (FIGS. 2 and 3A-3D). More
particularly, the first end 404a may operatively couple the rotor
216 to the transmission shaft 222 (FIG. 2), the driveshaft 226
(FIG. 2), and the drill bit 124 (FIG. 2) such that rotation of the
rotor 216 correspondingly rotates the drill bit 124. The second end
404b may comprise a connection configured to receive a
corresponding mating component (not shown), such as part of a rotor
jet nozzle retainer, a rotor catcher mechanism, or an electricity
transmitting rotary slip joint.
[0062] As illustrated, the rotor 216 provides and otherwise defines
an external profile 406 comprising a plurality of the rotor lobes
304 defined about the circumference of the rotor 216 in a helical
pattern. The profile 406 is configured to correspond to an internal
helical lobe profile of the stator 214 (FIGS. 3A-3D), as discussed
above, and extends at least partially between the first end 404a
and the second end 404b. The profile 406 comprises a multi-staged
helical configuration, where each stage is defined by the linear
distance of one full 360.degree. wrap of the rotor helix about the
body 402. While not shown in FIGS. 4A-4C, in some embodiments, the
rotor coating 318 of FIG. 3D may be applied to the profile 406,
without departing from the scope of the disclosure.
[0063] Since the rotor 216 can be manufactured by milling, turning,
swaging, cold rolling, pressure forming, or any combination of the
foregoing techniques, the rotor 216 may be thin or thick walled and
its stiffness can, therefore, vary significantly. This offers an
opportunity to selectively vary the stiffness of the rotor 216
relative to the mating stator 214 (FIGS. 2 and 3A-3D) and thereby
realize component benefits for the rotor 216 and the stator 214 and
also improve directional drilling control. More particularly, and
according to embodiments of the present disclosure, the internal
and/or external geometry of the rotor 216 may be varied along all
or a portion of its length in order to alter the stiffness of the
rotor 216 and/or optimize force balancing with respect to the
mating stator 214.
[0064] In FIG. 4A the rotor 216 is depicted as a solid, elongate
structure that may be formed of a metal, for example, such as
stainless steel. To vary the stiffness of the rotor 216, the
external geometry of the rotor 216 may be modified. More
particularly, in some embodiments, the mass of the rotor 216 may be
selectively modified by removing material at one or more locations
along the axial length of the body 402, as shown at locations 408a,
408b, 408c, and 408d. Removing the material from the one or more
locations 408a-d may be accomplished by a variety of machining
techniques, such as profiling, undercutting, and recessing. As
illustrated, for example, the diameter of the profile 406 of the
rotor lobe 304 positioned at the third location 408c may be altered
and otherwise profiled, undercut, or recessed over a specific
portion 410. In at least one embodiment, the portion 410 may be
selectively removed from one of the rotor lobes 304 along all or a
portion of its axial length as it helically-winds about the body
402. The portion 410 may be defined either by forming the decreased
dimension in the rotor 216 during manufacturing of the rotor 216
(i.e., swaging, milling, turning, etc.) or otherwise the portion
410 may be subsequently machined from the profile 406 to desired
tolerances following manufacture. Transition areas at the extents
of the profiled, undercut, or recessed zones are profiled in order
to minimize stress concentrations. As will be appreciated, by
removing mass from selected portions of the rotor 216, the
stiffness of the rotor 216 may be selectively decreased at those
locations.
[0065] In other embodiments, it may be desired to increase the
stiffness of the rotor 216. In such embodiments, one or more
stiffening elements 412 may be selectively secured to the outer
periphery of the body 402, such as is shown at the first and second
locations 408a,b. In some cases, the stiffening elements 412 may be
secured to the body 402 during manufacture of the rotor 216, and
thereby forming an integral part of the rotor 216 and the
associated profile 406. In other cases, however, portions of the
profile 406 may be removed by milling out or otherwise manufactured
to smaller dimensions and the stiffening elements 412 may
subsequently be attached to the rotor 216 at the locations where
the geometry of the profile 406 was altered.
[0066] The stiffening elements 412 may comprise a metallic or
non-metallic material or substance, which has specific physical
characteristics relative to that of the remaining or surrounding
portions of the body 402. Suitable materials that may be used as
stiffening elements 412 include, but are not limited to, lead,
steel, carbon-fiber, a polymer nano-composite, a liquid sealed in a
container, a piezoelectric fluid, a magneto-restrictive fluid, or
any combination thereof. In some embodiments, the stiffening
elements 412 may comprise weighting elements that may serve to
balance the weight of the rotor 216 in rotation and thereby provide
dynamic force balancing of the rotor 216.
[0067] In FIG. 4B, the rotor 216 is depicted as defining and
otherwise providing the rotor bore 312, which, as discussed above,
provides a bypass passageway through the rotor 216 for circulating
fluids. The stiffness of the rotor 216 in FIG. 4B may be altered by
varying the external geometry of the rotor 216 (i.e., the profile
406), as described above with reference to FIG. 4A, but also by
varying the internal geometry of the rotor bore 312 along all or a
portion thereof. In some embodiments, mass of the rotor 216 may be
selectively removed at discrete (short) or extended (long) portions
of the rotor bore 312, thereby decreasing the stiffness of the
rotor 216 at such discrete or extended portions. For instance, one
or more undercuts or internal recesses 414 (one shown) may be
defined in the rotor bore 312 at a corresponding one or more
discrete locations. The internal recess 414 effectively removes
material from the rotor 216 at that location, and thereby decrease
the stiffness of the rotor 216.
[0068] While the internal recess 414 is depicted in FIG. 4B as
being located at a discrete location, it is also contemplated
herein to extend the axial length or a large portion of the rotor
bore 312. In such embodiments, the internal recess 414 may
effectively enlarge the diameter of the rotor bore 312 and
simultaneously decrease the stiffness of the rotor 216. Moreover,
rather than providing 90.degree. corners, as illustrated, the
corners of the internal recess 414 may be rounded or angled (i.e.,
offset from 90.degree.), which may prove advantageous in enhancing
fatigue resistance during bending since abrupt structural changes
in structural components are more prone to cracking and/or
failure.
[0069] In other embodiments, mass of the rotor 216 may be
selectively removed by defining and otherwise providing one or more
profiles 416 (one shown) in the rotor bore 312 that may vary
axially according to a function across all or a portion of the
rotor bore 312. In the illustrated embodiment, the profile 416
tapers radially outward from left to right according to a linear
function, and thereby correspondingly increases the diameter of the
rotor bore 312 in the same direction. In other embodiments,
however, the profiles 416 may vary according to a polynomial
function that includes multiple diametrical variations or changes
along all or a portion of the axial length of the rotor bore 312.
In yet other embodiments, the profiles 416 may vary according to a
stepped or square function where the diameter of the rotor bore 302
increases and/or decreases in step-wise fashion along all or a
portion of the axial length of the rotor bore 312. In such
embodiments, rather than providing 90.degree. corners, one or more
of the steps of the stepped function may define rounded or angled
(offset from 90.degree.) corners, which, as mentioned above, may
enhance fatigue resistance.
[0070] As will be appreciated, the profile 416 removes mass from
the rotor 216 and thereby varies (decreases) its stiffness. In
downhole drilling motors, power units often fail near the bottom of
the stator 214 (FIGS. 2 and 3A-3D) since the weight of the
transmission shaft 222 (FIG. 2) directly affects rotation of the
rotor 216. By altering the stiffness of the rotor 216 toward the
second end 404b, it may be possible to negate the effects of the
mass of the transmission shaft 222 and the connections 218, 220
(FIG. 2).
[0071] In some embodiments, it may be desired to increase the
stiffness of the rotor 216 at select locations along its axial
length. In such embodiments, the profiles 416 may alternatively be
utilized to add mass to the rotor 216 by decreasing the diameter of
the rotor bore 312 according to a function (i.e., linear,
polynomial, square, etc.), without departing from the scope of the
disclosure. The stiffness of the rotor 216 may alternatively (or in
addition thereto) be increased by selectively positioning one or
more stiffening elements 412 within the rotor bore 312 at discrete
(short) or extended (long) internal recesses. In some embodiments,
for instance, as shown at location 408e, one or more stiffening
insert elements 412 in the form of cylindrical sleeves or strips
may be secured to the inner wall of the rotor bore 312, and thereby
effectively decrease the diameter of the rotor bore 312. In other
embodiments, however, one or more stiffening elements 412 may be
positioned within a corresponding internal recess 418 defined in
the rotor bore 312 such that the stiffening elements 412 do not
obstruct the flow passageway of the rotor bore 312.
[0072] In FIG. 4C, the rotor 216 includes a rotor mandrel 314,
which may be solid along its longitudinal axis or, as illustrated,
may include the rotor bore 312. The portion of the rotor 216
surrounding the rotor mandrel 314 may be referred to as a rotor
sleeve 420, which defines the profile 406 at its outer geometry.
The stiffness of the rotor 216 in FIG. 4C may be altered by varying
the external geometry of the rotor 216 (i.e., the profile 406), as
described above with reference to FIG. 4A, by varying the internal
geometry of the rotor bore 312 along all or a portion thereof, as
described above with reference to FIG. 4B, and by adding or
removing mass from one or both of the rotor mandrel 314 and the
rotor sleeve 420 at an interface 422 therebetween. The material of
the rotor mandrel 314 and the rotor sleeve 420 may be the same or
dissimilar. In embodiments where the materials are different, an
operator may selectively remove mass from one or both of the rotor
mandrel 314 and the rotor sleeve 420 to vary and otherwise optimize
the stiffness of the rotor 216 along its axial length.
[0073] Similar to the embodiment of FIG. 4B, the rotor bore 312 may
define one or more internal recesses 414 (one shown), as shown at
location 408f. As illustrated, the internal recess 414 may define a
specific cross-section profile. Moreover, similar to the embodiment
of FIG. 4B, the rotor bore 312 may also define one or more profiles
416 (one shown) that vary according to a function across all or a
portion of the rotor 216. The profile 416 in FIG. 4C varies
according to a square or stepped function, but could alternatively
vary according to a linear or polynomial function, without
departing from the scope of the disclosure. The overall stiffness
of the rotor 216 may further be altered by varying the geometry of
the interface 422 between the rotor mandrel 314 and the rotor
sleeve 420. The use of different materials that exhibit different
physical characteristics in the rotor mandrel 314 and the rotor
sleeve 420 may be taken into consideration in determining where to
alter the geometry of the interface 422 along the length of the
rotor 216. As illustrated, the thickness of the rotor sleeve 420
may be increased at the location 408h, but decreased along the
length of location 408g. Depending on the materials used for the
rotor mandrel 314 and its geometry, the rotor sleeve 420, and their
respective physical characteristics, such changes in geometry may
have the effect on either increasing or decreasing the stiffness of
the rotor at those locations 408g,h.
[0074] As will be appreciated, any of the features described in the
aforementioned embodiments of FIGS. 4A-4C to alter the geometry of
the rotor 216 may be employed and otherwise used in any
combination. For instance, one embodiment of the rotor 216 may
exhibit a single feature, such as the profile 416 that varies
according to a function. Another embodiment of the rotor 216 may
have mass removed from one or both of the rotor mandrel 314 and the
rotor sleeve 420. Another embodiment of the rotor 216 may include
stiffening elements 412 arranged at select locations or along all
or a portion of the outer periphery of the body 402 or the rotor
bore 312. Those skilled in the art will readily appreciate the
several variations that are feasible to selectively vary the
stiffness of the rotor 216 and balance its resulting rotational
weight with respect to the stiffness of the stator 214 (FIGS.
3A-3D), without departing from the scope of the disclosure.
[0075] In some embodiments, the geometry of the stator 214 and the
stator housing 306 (FIG. 3A) may also be altered to selectively
vary the stiffness of the motor 126. For instance, the outer
surface of the stator housing 306 may be selectively profiled or
externally undercut to vary the stiffness of the stator housing 306
relative to the stiffness of the rotor 216. As a result, the
overall robustness of the motor 126 may be altered and otherwise
optimized by modifying the stiffness of the rotor 216 in
conjunction with portions of the stator 214, such as the stator
housing 306.
[0076] Referring again to FIG. 1, with continued reference to FIGS.
2 and 4A-4C, in determining how to optimize the rotor stiffness and
dynamic weight balance of the rotor 216, a planar approach may be
adopted, where the change in wellbore azimuth 122 over the
effective length of the motor is considered to be negligible.
Wellbore interaction with the motor caused by irregular hole gauge
(possibly the result of hole wash-out, sloughing, caving or
cuttings build-up), may further exacerbate the mechanical loading
of the motor 126.
[0077] The mechanical properties of the rotor 126 may also be taken
into consideration in relation to the principal stresses that are
active upon the power unit 202 and the overall motor 126. Having
gained an understanding of the planar loading, the bending stress,
torsional stress and axial stress components, the effects of
downhole conditions and applied operating parameters may also be
considered. Such operating parameters include, but are not limited
to bending (e.g., weight on bit, stabilization, hole wash-out or
sloughing, hole caving or cuttings build-up etc.), applied string
RPM, string torque, and any combination thereof.
[0078] By considering the stress in both the rotor 216 and the
stator 214, an assessment can be made regarding their relative
stiffness in relation to the loading of the elastomer stator sleeve
308 (FIG. 3A), the stator coating 310 (FIG. 3B), and/or the rotor
coating (FIG. 3D) in both conventional and uniform elastomer
thickness power units (i.e., plain outer stator housing diameter or
variable outer stator housing diameter [flexed]). The stress
analysis may be based on standard mechanical stress calculations
and/or on specifically developed drilling industry related
calculations.
[0079] The maximum shear stress due to bending stress, torsional
shear, and axial stress can generally be determined using standard
calculations as follows:
1 2 ( Bending Stress + Axial Stress ) 2 + ( 4 .times. Torsional
Shear 2 ) Equation ( 1 ) ##EQU00001##
[0080] The maximum principal stress due to bending stress,
torsional shear, and axial stress may be determined as follows:
1 2 ( Bending Stress + Axial Stress ) + 1 2 ( Bending Stress +
Axial Stress ) 2 + ( 4 .times. Torsional Shear 2 ) Equation ( 2 )
##EQU00002##
[0081] Additionally, the Von Mises stress may be considered with
respect to component yield stress:
( Bending Stress + Axial Stress ) 2 + 3 .times. Torsional Shear 2 )
Equation ( 3 ) ##EQU00003##
[0082] The stresses in the rotor 216 and in the stator 214 are
paired with respect to desired relative rotor and stator stiffness
ratio criteria at discrete locations along their mated or paired
longitudinal length, and along/over their full/entire/overall mated
or paired longitudinal length, to achieve optimum mechanical
loading of the rotor and stator lobe forms, of any stator lining,
of any rotor or stator lobe form coatings, and of associated BHA
drilling tools. Accordingly, the rotor 216 and the stator 214 may
be physically modified at discrete locations along their mated or
paired longitudinal length or otherwise along their
full/entire/overall mated or paired longitudinal length to achieve
a desired relative stiffness and mass balancing.
[0083] As the rotor 216 rotates within the stator 214, centrifugal
load is created due to the mass of the rotor 216, which is resisted
by the elastomer lining (e.g., the stator sleeve 308 and/or the
stator coating 310) of the stator 214 along the length of the rotor
216. As mentioned above, the elastomer lining performs a radial
bearing function with respect to the rotating mass of the rotor
216. The transmission shaft 222 (FIG. 2) is connected to the lower
end of the rotor 216 and the upper end of the driveshaft 226 (FIG.
2). The transmission shaft 222 rotates due to the action of the
rotor 216 and an additional centrifugal load is created due to the
mass of the transmission shaft 222 and connections 218, 220. A
portion of this load is transferred to the rotor 216 and is
resisted by the internal stator lobe form (e.g. elastomer lining)
of the stator 214.
[0084] The movement of the rotor 216 and the transmission shaft 222
(FIG. 2) causes vibration and shock loading, which can be
detrimental to power production, can cause wear of the rotor 216
and the stator 214 and associated components. Torsional, lateral
and longitudinal axis vibration originating at the rotor 216 and
the stator 214 can affect the physical interaction between the
rotor 216 and the stator 214 (i.e. increased compressive loading
and rotor sliding and slippage), promoting fluid leakage at the
seal generated between the rotor 216 and the stator 214, thereby
reducing output torque and rotation.
[0085] Vibration loading originating from the drill bit 124 (FIGS.
1, 2 and 2A) and formation interactions tends to cause
elastomer/coating wear and degradation, which promotes elastomer or
coating overheating, hardening, and cracking. Vibration and shock
loading imparted into the stator elastomer by the rotor 216 tends
to cause hysteresis within the elastomer. During the hysteresis
process, mechanical energy imparted into the elastomer manifests
itself as internally generated heat within the elastomer, which is
not completely dissipated. Over time, heat accumulates within the
elastomer to an extent where the physical characteristics of the
elastomer are detrimentally affected. Such vibration and shock
loading can also cause interference in terms of electronic or
pressure-based data transmission through/across motors and to/from
measurement-while-drilling (MWD) and/or logging-while-drilling
(LWD) tools within the BHA.
[0086] Only a portion of the load caused by the rotating mass of
the transmission shaft 222 is resisted by the bearings in the
bearing section 206 (FIG. 2). The elastomer lining of the stator
214, therefore, performs a radial bearing function with respect to
the action of the rotating mass of the rotor 216 plus a portion of
the load caused by the rotating mass of the transmission shaft 222
and the connections 218, 220.
[0087] FIG. 5 is a cross-sectional end view of an example
embodiment of a multi-lobe power unit 202 that schematically
depicts the geometric interaction between the rotor 216 and the
stator 214. Eccentricity (Eccr) can be related to the radial
movement of the central axis 502a of the rotor 216 relative to the
central axis 502b of the stator 214 as the central axis 502a of the
rotor 216 moves during precession or nutation.
[0088] In FIG. 5, the geometric interaction between the rotor 216
and the stator 214 includes a major diameter D.sub.maj and a minor
diameter D.sub.min. The major diameter D.sub.maj is defined by the
diameter of a circle that radially circumscribes the outermost
points of the stator lobes 302, at the lobe troughs A. The minor
diameter D.sub.min is defined by the diameter of a circle that
circumscribes the radially innermost points of the stator lobes, at
the lobe crests B. The eccentricity of a mated rotor 216 and stator
214 pair is a function of the major diameter D.sub.maj and the
minor diameter D.sub.min. More particularly, the eccentricity of a
mated rotor 216 and stator 214 pair, where the stator 214 has more
than one lobe 302, equals:
( D maj - D min ) 4 Equation ( 4 ) ##EQU00004##
[0089] The centrifugal/inertia force (F.sub.c) of the rotor 216 is
equal to the mass (M) of the rotor 216 multiplied by the rotational
speed squared (v.sup.2), multiplied by the eccentricity (Eccr), as
shown as follows:
F.sub.c=M.times.v.sup.2.times.Eccr Equation (5)
[0090] In determining how to optimize the rotor stiffness and
dynamic mass balance of the rotor 216, vibration caused by the
motor 126 may also be considered. There are various aspects to
consider when considering vibration caused by a downhole drilling
motor such as, but not limited to, the mass of the rotor 216,
eccentric movement of the rotor 216 within the stator 214 (i.e.,
eccentric rotation produces an imbalance), eccentric rotation speed
(i.e., nutation), the fit between the rotor 216 and the stator 214
(i.e., the mating fit clearance or compression fit, rotating/fit
and whether there is sliding/rubbing tendency, angular and parallel
misalignment, etc.), the effects of the transmission shaft 222
(FIG. 2) (i.e., its mass and potential sub frequencies from moving
elements), the effects of the driveshaft 226 (FIG. 2) (i.e., its
mass and potential sub frequencies from moving elements), the
adjustable housing offset (i.e., resulting rotor 216 offset to
driveshaft 226 and rotor 216 loading against stator 214 lining
along length), drilling fluid flow paths/pressure pulsation (i.e.,
progressing fluid cavities), any external damping (i.e., motor 126
geometry, formation contact, annular fluid, associated BHA
components), and any combination thereof.
[0091] When the power unit 202 functions, there are two rotations
present: clockwise rotation of the rotor 216 about its central axis
502a (FIG. 5), and anti-clockwise precessing rotation of the rotor
216 within the stator 214. Only clockwise rotation is transmitted
to the drill bit 124 (FIGS. 1 and 2), the transmission section 204
(FIG. 2) negates the precession effects of the rotor 216.
[0092] Motor vibration mainly results from the mass, amount of
eccentricity, and eccentric rotation speed of the rotor 216 within
the stator 214. As the rotor 216 rotates inside the stator 214,
these factors determine the amount of vibration that the power unit
202 may generate. The greater the mass, the larger the eccentricity
and the greater the eccentric rotation speed and, consequently, the
higher the resulting vibration.
[0093] The lobe profiles 302, 304 (FIGS. 3A-3D) are the same on the
rotor 216 and the stator 214, they are produced on different
effective pitch circle diameters, the rotor 216 having one less
lobe 304 than the stator 214. The lobe 302, 304 form is designed to
maximize rolling, minimize sliding and minimize mechanical loading
(amplitude) as the rotor lobes 304 mesh at speed with the stator
lobes 302. As the number of lobes 302, 304 increase, the depth and
eccentricity of the lobe 302, 304 decrease, the frequency increases
and the amplitude of loading tends to reduce.
[0094] The greatest centrifugal forces are generated at the
eccentric rotation speed since the largest percentage of mass in
the motor 126 is the rotor 216 and the eccentric rotation of the
rotor 216 has the largest radius of gyration with respect to the
offset rotating transmission shaft 222 (FIG. 2) and the concentric
rotating driveshaft 226 (FIG. 2). The eccentric rotation speed of
the rotor 216 is equal to the output RPM of the driveshaft 226
multiplied by the number of rotor lobes 304. The frequency (Hz) of
the eccentric rotor 216 loading of the stator 214 equals the output
RPM divided by 60 and multiplied by the number of rotor lobes 304.
This defines the number of times a stator lobe 302 is loaded in a
given unit of time.
[0095] As the number of lobes 302, 304 (FIGS. 3A-3D) increases, the
loading frequency increases. Eccentric rotation speed of the rotor
216 tends to coincide with the strongest vibrations. Lesser
vibration peaks tend to occur at integer orders of
magnitude/multiples of eccentric rotation speed of the rotor 216,
related to the number of rotor lobes 304. The resulting frequencies
are relatively low (i.e., less than 500 Hz). The more significant
vibration peaks (amplitudes) typically tend to occur at less than
100 Hz, when free running or loaded when on-bottom drilling.
[0096] Smaller diameter motors of less than 43/4 inch outer
diameter with low lobe numbers tend to have relatively high output
speeds. The eccentricity and mass of the smaller rotors is less,
operating frequencies tend to be relatively high, and amplitudes
are relatively low.
[0097] Vibration caused by drill bit 124 interactions with the
formation generally has different amplitude-frequency signatures to
the vibration generated by the motor 126. Torsional, lateral, and
longitudinal axis vibration originating at the drill bit 124 can
affect the physical interaction between the rotor 216 and the
stator 214 (i.e. increased compressive loading and rotor sliding
and slippage), promoting fluid leakage at the seal generated
between the rotor 216 and the stator 214, thereby reducing output
torque and rotation. The vibration loading from the drill bit 124
tends to cause elastomer/coating wear and degradation, which
promotes elastomer or coating overheating, hardening and
cracking.
[0098] The centrifugal loading produced by the rotor 216 is set
with respect to desired loading criteria to achieve optimum dynamic
loading of the rotor and stator lobe forms, of any stator lining,
of any rotor or stator lobe form coatings, and of associated BHA
drilling tools.
[0099] The speed and torque of the drill string 120 (FIG. 1)
delivered to the motor 126 during drilling may also have a direct
effect on how the power unit 202 actually functions in terms of its
input-output operating envelope, its mechanical loading and
efficiency. As will be appreciated, this affects the internally
generated vibration signature of the motor 126.
[0100] Drilling fluid characteristics also affect the vibration
signature of the motor 126 and interact with the rotor 216 and the
stator 214 during operation. Hydraulic loading of the elastomer
lining (e.g., the stator sleeve 308 and/or the stator coating 310)
of the stator 214 is inherent to the progressing cavity power unit
202. The rotor 216 is effectively radially sealed and physically
supported within the stator 214. Pressurized drilling fluid (or
another circulating fluid) impinges on the upper end of the rotor
216/stator 214 combination. This results in hydraulic downthrust of
the rotor 216. As the pressurized fluid passes (progresses) between
the rotor 216 and the stator 214, the rotor 216 is forced to
rotate, and allowable eccentric movement of the rotor 216 results
in the rotor 216 precessing within the stator 214. During
operation, the pressure inside the stator 214 may not be linear.
Under some circumstances, for instance, the internal pressure
builds towards the lower or downhole end of the stator 214. This is
in addition to the loading effect of the rotating transmission
shaft 222 (FIG. 2). According to the present disclosure, the
geometry, stiffness, and/or mass of the rotor 216 may be varied to
compensate for the hydraulic downthrust and internal pressure,
which, to some extent, can negate or otherwise mitigate centrifugal
loading and internal pressure.
[0101] As discussed above, the rotor 216 is connected to a
transmission shaft 222 (FIG. 2), which is connected to a driveshaft
226 (FIG. 2). Thrust bearings within the bearing section 206 (FIG.
2) resist the downthrust load on the rotor 216 and downward
movement. Mechanical loads are generated at the drill bit 124 as it
engages the underlying formation interface and such loads are
transmitted from the drill bit 124 to the lower end of the stator
214 via the driveshaft 226 and the transmission shaft 222. The
rotating mass of the upper end of the transmission shaft 222 is
essentially constrained by the lobe form 302 (FIGS. 3A-3D) of the
stator 214 and, therefore, by any elastomer lining (e.g., the
stator sleeve 308 and/or the stator coating 310) of the stator 214.
The lower end of the stator 214 sustains higher mechanical loading
from the effect of the transmission shaft 222 as compared to its
top end. According to the present disclosure, the mass of the rotor
216 may be varied at discrete locations along its longitudinal
length, and over its full/entire/overall longitudinal length. As
will be appreciated, this may help compensate for the downthrust
loads, and internal pressure, and the loads received from the drill
bit 124, and thereby optimize the loading at the stator bore
(lining or coating) and between the rotor 216 and stator 214 lobes
at their interface, meshing, mating (seal) area.
[0102] The relative stiffness of the rotor 216 and the stator 214
and the mass of the rotor 216 are considered in relation to
optimizing the functioning and reliability of the motor power unit
202, and the functioning of the full motor assembly, to benefit the
functioning of associated BHA equipment, and to enhance directional
drilling modelling, planning, and control.
[0103] Predictive mechanical analysis software is used to consider
the behavior of the drill string 120 (FIG. 1) and BHA within the
three-dimensional wellbore. The drill string 120 and BHA components
are considered as an overall system, as individual drilling tools,
and as sub-divided three-dimensional elements.
[0104] Many aspects are considered based on BHA component and
physical characteristics. These are modelled in terms of various
parameters being coincident upon the BHA, including wellbore
curvature and inclination, side forces at component or element
wellbore contact points, bending, torsional, longitudinal and
combined stresses, in relation to directional trajectory tendency
(azimuth and inclination), directional targets, critical rotational
speeds, critical buckling loads and torque and drag (relative to
localized wellbore tortuosity and doglegs). Manipulation or tuning
of the stiffness, and mass balance of the power unit 202 of the
motor 126 allows for the modelling and accurate physical
configuration of the motor assembly in terms of planning for
optimum motor performance, enhancing the reliability of associated
BHA components and enhancing BHA directional trajectory
control.
[0105] While the foregoing disclosure and description is related to
downhole drilling motors, such as positive displacement motors or
"mud motors," those skilled in the art will readily appreciate that
the principles discussed herein are equally applicable to other
types of motors and pumps including, but not limited to,
progressing cavity pumps used at a surface location or downhole.
With surface-mounted or downhole progressing cavity pumps, for
instance, the principles of the present disclosure may be applied
to mass balance the various associated components of the pump(s),
without departing from the scope of the disclosure.
[0106] Therefore, the disclosed systems and methods are well
adapted to attain the ends and advantages mentioned as well as
those that are inherent therein. The particular embodiments
disclosed above are illustrative only, as the teachings of the
present disclosure may be modified and practiced in different but
equivalent manners apparent to those skilled in the art having the
benefit of the teachings herein. Furthermore, no limitations are
intended to the details of construction or design herein shown,
other than as described in the claims below. It is therefore
evident that the particular illustrative embodiments disclosed
above may be altered, combined, or modified and all such variations
are considered within the scope of the present disclosure. For
example, a separate mechanism such as a dual mass flywheel or
centrifugal pendulum absorber may be employed within a motor to
improve rotor mass balance and reduce torsional fluctuations and
vibration.
[0107] Embodiments disclosed herein include:
[0108] A. A method of manufacturing a power unit for a downhole
drilling motor that includes fabricating a stator that provides two
or more stator lobes that define an internal profile, fabricating a
rotor that provides at least one rotor lobe that defines an
external profile that precesses within the internal profile during
operation, varying at least one of an external geometry and an
internal geometry of the rotor along all or a portion of the rotor
to alter a stiffness of the rotor and thereby optimize stiffness
with respect to the stator, and rotatably positioning the rotor
within the stator.
[0109] B. A power unit for a downhole drilling motor that includes
a stator that provides two or more stator lobes that define an
internal profile, and a rotor rotatably positioned within the
stator and providing at least one rotor lobe that defines an
external profile that precesses within the internal profile during
operation, wherein at least one of an external geometry and an
internal geometry of the rotor is varied along all or a portion of
the rotor to alter a mass of the rotor and thereby optimize force
balancing with respect to the stator.
[0110] Each of embodiments A and B may have one or more of the
following additional elements in any combination: Element 1:
wherein varying the external geometry of the rotor comprises
altering a dimension of the external profile. Element 2: wherein
varying the external geometry of the rotor comprises securing one
or more stiffening elements to the external profile. Element 3:
wherein the rotor defines a rotor bore and varying the internal
geometry of the rotor comprises defining one or more internal
recesses in the rotor bore. Element 4: wherein the rotor defines a
rotor bore and varying the internal geometry of the rotor bore
comprises defining one or more profiles in the rotor bore that
extend axially along all or a portion of the rotor bore according
to a function. Element 5: wherein the function is selected from the
group consisting of a linear function, a polynomial function, a
square function, and any combination thereof. Element 6: wherein
the rotor defines a rotor bore and varying the internal geometry of
the rotor comprises positioning one or more stiffening elements
within the rotor bore. Element 7: further comprising selectively
positioning the one or more stiffening elements within the rotor
bore to optimize the stiffness with respect to the stator. Element
8: further comprising profiling an outer surface of the stator to
selectively vary a stiffness of the stator. Element 9: wherein the
rotor includes at least one of a rotor mandrel and a rotor sleeve,
and wherein varying at least one of the external geometry and the
internal geometry of the rotor comprises varying a stiffness of at
least one of the rotor, the rotor mandrel, and the rotor sleeve
relative to a stiffness of a stator housing and a stator lining.
Element 10: wherein the rotor comprises a rotor sleeve that defines
the external profile and a rotor mandrel positioned within the
rotor sleeve and defining a rotor bore, the method further
comprising varying a geometry of an interface between the rotor
mandrel and the rotor sleeve to alter the stiffness of the rotor
and thereby optimize a force balancing with respect to the stator.
Element 11: wherein the rotor comprises a rotor sleeve that defines
the external profile and a rotor mandrel positioned within the
rotor sleeve and defining a rotor bore, the method further
comprising mass balancing at least one of the rotor, the rotor
sleeve, and the rotor mandrel.
[0111] Element 12: wherein the external geometry of the rotor is
varied by altering a dimension of the external profile. Element 13:
wherein one or more stiffening elements are secured to the external
profile to vary the external geometry of the rotor. Element 14:
wherein the rotor defines a rotor bore and one or more internal
recesses are defined in the rotor bore to vary the internal
geometry of the rotor. Element 15: wherein the rotor defines a
rotor bore and one or more profiles are defined in the rotor bore
and extend axially along all or a portion of the rotor bore
according to a function to vary the internal geometry of the rotor.
Element 16: wherein the function is selected from the group
consisting of a linear function, a polynomial function, a square
function, and any combination thereof. Element 17: wherein the
rotor defines a rotor bore and one or more stiffening elements are
positioned within the rotor bore to vary the internal geometry of
the rotor. Element 18: wherein the one or more stiffening elements
comprise weighting elements that optimize the force balancing with
respect to the stator. Element 19: wherein the rotor comprises a
rotor sleeve that defines the external profile and a rotor mandrel
is positioned within the rotor sleeve. Element 20: wherein a
geometry of an interface between the rotor mandrel is varied to
alter a stiffness of the rotor with respect to a stiffness of the
stator. Element 21: wherein an outer surface of the stator is
profiled to selectively vary a mass of the stator. Element 22:
wherein the stator comprises a stator housing and a stator sleeve
positioned within the stator housing and defining the internal
profile, and wherein the stator sleeve comprises a material
selected from the group consisting of a metal, a metal alloy, a
polymer and any combination thereof. Element 23: wherein the stator
comprises a stator housing that defines the internal profile and a
stator coating applied to the internal profile and comprising an
elastomer or a rubber.
[0112] By way of non-limiting example, exemplary combinations
applicable to A and B include: Element 6 with Element 7; Element 15
with Element 16; Element 17 with Element 18; and Element 19 with
Element 20.
[0113] The systems and methods illustratively disclosed herein may
suitably be practiced in the absence of any element that is not
specifically disclosed herein and/or any optional element disclosed
herein. While compositions and methods are described in terms of
"comprising," "containing," or "including" various components or
steps, the compositions and methods can also "consist essentially
of" or "consist of" the various components and steps. All numbers
and ranges disclosed above may vary by some amount. Whenever a
numerical range with a lower limit and an upper limit is disclosed,
any number and any included range falling within the range is
specifically disclosed. In particular, every range of values (of
the form, "from about a to about b," or, equivalently, "from
approximately a to b," or, equivalently, "from approximately a-b")
disclosed herein is to be understood to set forth every number and
range encompassed within the broader range of values. Also, the
terms in the claims have their plain, ordinary meaning unless
otherwise explicitly and clearly defined by the patentee. Moreover,
the indefinite articles "a" or "an," as used in the claims, are
defined herein to mean one or more than one of the elements that it
introduces. If there is any conflict in the usages of a word or
term in this specification and one or more patent or other
documents that may be incorporated herein by reference, the
definitions that are consistent with this specification should be
adopted.
[0114] As used herein, the phrase "at least one of" preceding a
series of items, with the terms "and" or "or" to separate any of
the items, modifies the list as a whole, rather than each member of
the list (i.e., each item). The phrase "at least one of" allows a
meaning that includes at least one of any one of the items, and/or
at least one of any combination of the items, and/or at least one
of each of the items. By way of example, the phrases "at least one
of A, B, and C" or "at least one of A, B, or C" each refer to only
A, only B, or only C; any combination of A, B, and C; and/or at
least one of each of A, B, and C.
* * * * *